Bearing elements are generally known which comprise a steel back, a substrate layer and a sliding layer (or overlay or running layer). These are commonly used in engines, for example as crankshaft and/or camshaft supporting bearings and big end bearings and small end bushings in connection rods. They may also be used as thrust washers (axial bearings).
The sliding layer is made from a sliding layer material, which may be a metal layer, comprising, for example, lead, tin, bismuth, silver, copper alloys or aluminium alloys. The sliding layer may be applied to a substrate by galvanic processes, vaporisation or mechanical plating. Alternatively, the sliding layer material may be a non-metal material comprising an artificial resin base, or matrix, and an additive for enhancing the load carrying capacity and/or wear resistance of the bearing.
It is generally known that wear of the sliding layer material can lead to exposure of the substrate material to which the sliding layer material is applied. This can lead to failure of the bearing element due to seizure.
Research in the field of bearing elements has resulted in the use of a wide range of compositions of sliding layer materials, many incorporating relatively harder particles, particularly where the bulk of the sliding layer material is an artificial resin base or matrix, with the intention of providing the bearing element with enhanced wear resistance. There has some been interest in the use of iron oxides in sliding layer materials as discussed in the following prior art patents and applications.
Published international patent application number WO9738046 in the name of Glyco Metall Werke describes the use of sliding layer materials predominantly consisting of thermoplastic fluoropolymers to which iron oxide is added to increase the resistance to wear of the sliding layer material.
Published Japanese patent application JP2005201289 to Toyota Motor Corp. et al. claims to provide a composite sliding layer formation which does not require preheating a base material. The application refers to the use of hard particles, including iron oxide (Fe2O3), which may be included in the sliding layer to act as a friction modifier and wear inhibitor.
U.S. Pat. No. 8,551,569 granted to Federal-Mogul Wiesbaden GmbH describes a method for producing a sliding element having a metal base material that is provided with a sliding layer having a thickness of more than 50 μm. To make the sliding element, the base material is coated with a paste of the sliding layer material that contains at least one high-temperature polymer in addition to a fluoropolymer, and the coated base material is subjected to a thermal treatment. The paste may contain a wear-reducing additive, such as iron (III) oxide.
Published international patent application number WO2010076306 in the name of Federal-Mogul Wiesbaden GmbH describes a sliding element having a substrate and a sliding layer material applied to the substrate. The sliding layer material comprises at least one cross-linkable binder or at least one high-melting thermoplastic material or is of a material that contains a matrix of at least one high-melting thermoplast material or at least one “Duroplast” material. The sliding layer material contains Fe2O3 with a preferred volume percentage fraction of 0.1 to 15 vol. %. The substrate to which the sliding layer material is applied has a particularly preferred surface roughness (RZ) of 3 to 8 μm where RZ is understood to be the surface roughness according to DIN EN ISO 4287:1998.
Federal-Mogul Wiesbaden GmbH sells crankshaft bearings and conrods under the trade name “IROX” which comprise a substrate and an overlay (sliding layer material) consisting of polyamide imide (PAI) and Fe2O3 particles.
The applicant for the present invention has found that existing attempts to use iron oxides in sliding layer materials have resulted in bearing elements which have a number of limitations and which are not optimised for certain applications in automotive environments. For example, known bearing elements comprising Fe2O3 are prone to some or all of the following drawbacks: sagging of the sliding layer material; variation of the sliding layer material thickness across the surface of the substrate; and contamination of the sliding layer material, for example contamination with grit particles which have been used to roughen the surface of the substrate prior to application of the sliding layer material.
The applicant has also found that in known bearing elements, the surface of the substrate to which the sliding layer material is applied is relatively coarse, having, a particularly preferred surface roughness (Rz) of 3 to 8 μm, or higher. This may result in the formation of large exposed peaks or asperities of the underlying bearing materials as the sliding layer material wears. This may lead to excessive metal-to-metal contact and is highly undesirable.